Facebook in Firefox 15.0.1

When I try to log in to mi Facebook account, as soon as I open the browser and go to facebook.com i¿I received a message saying:

Update Your Browser
You’re using a web browser that isn’t supported by Facebook.
To get a better experience, go to one of these sites and get the latest version of your preferred browser:

then when i tried to download the latest version is the same firefox 15.0.1 that doesnt work

When I try to log in to mi Facebook account, as soon as I open the browser and go to facebook.com i¿I received a message saying:
Update Your Browser
You’re using a web browser that isn’t supported by Facebook.
To get a better experience, go to one of these sites and get the latest version of your preferred browser:
then when i tried to download the latest version is the same firefox 15.0.1 that doesnt work

Your Firefox is identifying itself as Firefox 3.6.27.
Unless you are intentionally sending that information, it probably indicates an obsolete setting. You can try to clear it using the steps in this article under "Other Causes": https://support.mozilla.org/en-US/kb/websites-say-firefox-outdated-or-incompatible#w_other-causes
Any luck?